1. Field of the Invention
The present invention is directed toward a catalyst that is particularly useful in reforming of tars present in syngas generated through biomass gasification. The catalyst comprises a physical mixture of particles of a catalytic material, such as NiO, and particles of a char. The char acts as a support for the catalytic material, but can also participate in the reforming process by providing a source of carbon.
2. Description of the Prior Art
Biomass gasification is considered a promising technology for production of sustainable fuels. Biomass gasification is a process in which biomass undergoes incomplete combustion to produce a gas product called syngas that comprises mainly of H2, CO, CH4, CO2, and N2 (if air is used in combustion) in various proportions. Biomass gasification has many advantages over direct combustion. It converts low-value feedstocks to high-quality combustible gases, which can be not only directly burned or used for electricity generation but also turned into liquid transportation fuels. However, tars are produced along with syngas during biomass gasification.
“Tar” is a generic term describing all organic compounds present in syngas except for the lighter gaseous hydrocarbons, such as methane. Tars can condense to more complex structures in pipes, filters, or heat exchangers of downstream equipment and processes, which may cause mechanical breakdown of the entire system. Tars may also deactivate catalysts in the refining process. Tar content varies from about 0.5 to 100 g/m3 in biomass gasification syngas depending on the type and design of the gasifier, feedstock used, and operating conditions. Most applications of syngas require tar content below 50 mg/m3; therefore, it is essential to reduce the level of tars to enable widespread utilization of syngas.
Several approaches for tar removal, such as physical treatment, thermal cracking, plasma-assisted cracking, and catalytic reforming, have been reported in the relevant technical literature. Among these, catalytic reforming is considered the most promising in large-scale applications because of its fast reaction rate and reliability and its ability to increase the quantity of useable gases such as CO and H2 in syngas.
Various types of catalysts including calcined rocks, zeolites, iron ores, alkali metals, Ni-based catalysts, and precious metals have been studied for their usefulness in tar removal in biomass gasification. For catalytic reactivity and economic reasons, Ni-based catalysts are considered the most promising for tar removal and syngas reforming. Nickel catalysts can be usually supported by metal oxides (e.g., Al2O3 and MgO) or natural materials (e.g., dolomite and olivine). Some of these supports are expensive, and the catalyst preparation steps involve impregnation and calcination of the nickel onto the support, both of which are time and energy consuming. These factors limit extensive application of Ni-based catalysts for tar removal.